Resolve "Add tolerations to kubernetes executor"

What does this MR do?

Allows tolerations to be added to pods created by the Kubernetes executor, so such pods can be scheduled onto tainted nodes.

Why was this MR needed?

See #2681 (closed).

Are there points in the code the reviewer needs to double check?

• Are the omitempty/omitzero parts correct?
• The descriptions in the KubernetesToleration struct are taken straight from the Kubernetes documentation. Is this OK?

Does this MR meet the acceptance criteria?

• Documentation created/updated
• Tests
  • Added for this feature/bug
  • All builds are passing
• Branch has no merge conflicts with master (if you do - rebase it please)

What are the relevant issue numbers?

Closes #2681 (closed)